The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
The use of laser rangefinders is known in a variety of industries. These devices use a laser beam to determine the distance to a diffuse object (otherwise known as “Time of Flight” measurement). The laser diode emits light pulses of about 35-45 nanoseconds, which hit the target and a small amount reflects off the target through diffuse scatter. This small amount of light is optically detected by a sensor and is amplified sufficiently to register in multiple circuits containing a high speed chronometer that measures the time it took for the light to return from the target. This is then translated into the distance to yield a range measurement. An alternative to Time of Flight measurement involves using a triangulation method of calculation. The triangulation laser shines a laser on the subject and exploits a camera to look for the location of the laser dot. The distance between the camera and the laser emitter is known. The angle of the laser emitter corner is also known. The angle of the camera corner can be determined by looking at the location of the laser dot in the camera's field of view. These three pieces of information fully determine the shape and size of the triangle and give the location of the laser dot corner of the triangle.
Difficulties arise however when the target is reflective, as the detector will fail to register a reading. This is because the laser is reflected directly off the reflective surface and so there is no dispersion of light when it hits a reflective surface, thus, a measurement can not be detected. By extension, if an object fluctuates between reflective and non-reflective, the detector will fluctuate between registering a reading, and registering nothing.
As an example, in conventional metal coating processes, a pair of opposed air knives are used to remove excess coating material from the strip/sheet of metal passing therebetween. In most circumstances, better control of the coating can be achieved when the air knife is close to the strip. However, minor buckling and trembling of the strip as it passes the air knife can cause the strip to hit the air knife, blocking it with the coating material, and resulting in poor coating consistency (or even damage to the strip). This results in the line having to be shut down to clear the air knife. Due to the requirement of small margins of error to adhere to industry specifications, inconsistent coating thickness and unexpected shutdowns can have a significant financial impact.
In order to avoid the aforementioned problems a number of solutions have been previously attempted. These typically involve complicated algorithms that utilise many control factors, such as strip velocity and air pressure to determine the strip distance. Due to the many constraints on the algorithms, these calculations have large margins for error and often result in erroneous strip distances being calculated. This is primarily due to the fact that they are numerical back-calculations. There is currently no real-time physical measurement available in the industry that covers the full range of equipment movement.
Other control methods have utilised a measurement of the distance between the air knife and the coated surface in order to identify and correct for when the strip is too close to the air knife. These have typically utilised either ultrasonic or induction sensors. However, ultrasonic sensors can fail to report a distance when used on very hot surfaces, and induction sensors have a limited range and do not cover the full scope of operating conditions.
Alternatively, lasers have been selected to measure the distance. However the lasers must be arranged to either measure the distance to a reflective surface only, or a non-reflective surface, only. As the strip moves past the knife, the coated surface will often be molten, and thus is reflective, but there are times in which the coating will solidify resulting in a matte, non-reflective surface. This results in inconsistent and incomplete data. To date, lasers have not been able to be used on both reflective and non-reflective surfaces interchangeably.
The present invention seeks to overcome, or at least ameliorate, one or more of the deficiencies of the prior art mentioned above, or to provide the consumer with a useful or commercial choice.
Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.